1. Field of the Invention
The present invention generally relates to devices for producing electrical signals to be inductively coupled to a conductor, and more particularly to an amplifier having an application as a driver for an induction coil which may be used in locating buried cables and the like.
2. Description of the Prior Art
Induction coils are well known in the art, and are used to generate alternating currents or high voltage pulses in conductors, as well as to create high voltage signals from low-voltage current, as is accomplished in a standard transformer. The present invention is not directed to induction coils per se, but rather relates to circuits for driving such coils.
One particular application for induction coils relates to location of previously buried objects. It often becomes necessary to locate defective or damaged cables, pipes, etc., in order to repair or replace them. Conversely, it is important to know the approximate vicinity of such items in order to avoid disturbing them when digging or excavating for other purposes. Several methods employing inductive coupling of signals have been devised for searching for such objects.
For example, underground cables, such as telephone and CATV cables, surface at various locations in terminal boxes known as pedestals. An amplified signal source may be inductively coupled to a given wire or wire pair at the pedestal. The wire acts as an antenna, re-radiating the signal along the full length of the cable. A receiver unit may then be used above ground to trace the path of the buried cable.
It is possible to directly connect the signal source to the cable where a bare wire is exposed, but this is undesirable as it may result in interference with signals or conversations on the cable. Moreover, direct connection creates a potential shock hazard, and is further unsuitable in instances where no bare wire is exposed. Inductive coupling of the signal to the wire is thus preferable.
The assignee of the present invention, Minnesota Mining & Manufacturing Co. of St. Paul, Minn., markets several products which incorporate the latter technique. 3M's Dynatel 500 Cable Locator and Dynatel 573 Fault Locator each employ inductive coupling to send the source signal through the cable. The transmitter unit of these devices may simply be placed above the cable, as there is an internal antenna within the transmitter housing which acts as an inductive coil. Alternatively, 3M's Dyna-Coupler accessory may be used to isolate a single wire for inductive coupling ("Dynatel" and "Dyna-Coupler" are registered trademarks of 3M).
A similar technique employing inductive coupling is used in passive electronic marker systems. Passive markers are basically wire coils surrounded in a protective envelope, which are then buried adjacent the cable, pipe, or other object to be marked. There is no power supply in a passive marker, and they are self-contained, with no external, accessible connections. 3M markets several kinds of passive markers for different applications as part of its ScotchMark Electronic Marker System ("ScotchMark" is a registered trademark of 3M). Two of these markers are described in the following U.S. patents which are hereby incorporated by reference: U.S. Pat. Nos. 4,334,227 issued to B. Marks, and 4,712,094 issued to J. Bolson.
Passive markers are activated by radiating a signal toward the ground in the area where the marker is expected to be found. The signal is emitted via an inductive coil held very close to the surface. When the coil is directly over the passive marker (which is itself an inductive coil), a resonance is created, and the passive marker re-emits the signal. A second coil within the unit acts as a receiver, and detects the reradiated signal, alerting the service technician with an audible tone or other indicator means. A good explanation of one such transmitter/receiver device may be found in Canadian Pat. No. 993,516, hereby incorporated by reference, which is based on a U.S. patent application, Ser. No. 523,263 (filed Nov. 13, 1974), now abandoned.
Passive markers are usually color-coded according to the particular type of utility line they mark. Specifically, gas line markers are yellow; telephone cable markers are orange; sewage tunnel markers are green; water pipe markers are blue; and power supply markers are red. Similarly, the inductive coil in a passive marker is "coded" by tuning the coil for a specific resonant frequency. Five distinct frequencies have been designated: 83.0 kHz for gas; 101.4 kHz for telephone; 121.6 kHz for sewage; 145.7 kHz for water; and 169.8 kHz for power. In this manner, a service technician searching for, say, a gas line, cannot accidentally activate a telephone marker since his transmitter will only be sending out an 83 kHz signal which will not affect the telephone marker tuned for 101.4 kHz.
Although the foregoing convention for tuning markers has obvious advantages, it also creates several problems. First, it requires that the induction coil in the transmitter be tuned to same resonant frequency as that of the passive marker. Tuning the transmitter coil minimizes power losses in the transmitter and maximizes power output at the desired frequency. This is normally accomplished by including a variable capacitor in series with the transmitter's induction coil (forming an LC circuit). In so tuning the transmitter, however, the usefulness of the device becomes limited to that particular frequency. For example, suppose the service technician from the preceding paragraph wanted to search for both gas and telephone lines in the designated area. He would have to use two different transmitter/receivers, separately scanning the area, and thus doubling his effort. Transmitters have been designed which are capable of emitting more than one frequency to an induction coil. The two different frequencies are not, however, transmitted simultaneously.
FIG. 1 is a representation of a generalized prior art transmitter similar in nature to the Dynatel 500 and 573 Cable Locators. A switch 1 directs the appropriate frequency signal 2, 3 or 4 (f.sub.1, f.sub.2 or f.sub.3) through a linear amplifier 5 to one of three induction coils 6, 7 and 8. Each induction coil is tuned by provision of separate LC circuits (L.sub.1 C.sub.1, L.sub.2 C.sub.2 and L.sub.3 C.sub.3). Such a device is still inadequate with regard to detection of different frequency markers since it emits only a single-frequency signal at any given time. The transmitter of FIG. 1 could be altered to provide a multiple-frequency signal through a conventional linear amplifier(s) to all three coils simultaneously, or to simply provide the multiple-frequency signal to a single, non-tuned induction coil. In either case, however, power losses are excessive and unacceptable, in light of the fact that these units are typically portable with internal battery packs. Additionally, the presence of multiple coils creates interference or shorting therebetween causing additional power losses. It would, therefore, be desirable and advantageous to devise a driver for an induction coil which could simultaneously emit multiple frequencies with high efficiency power consumption.
Accordingly, the primary object of the present invention is to provide an amplifier having a particular application as a driver for an induction coil.
Another object of the invention is to provide such an induction coil driver which supplies a single- or multiple-frequency source.
Still another object of the invention is to provide an induction coil circuit which may drive the coil in a non-resonant mode.
Yet another object of the invention is to provide an induction coil driver with a high efficiency over a wide frequency range for use in a portable, battery-power supplied unit.